202 Frequency Synthesizer Fault Finding TM8100/TM8200 Service Manual
© Tait Electronics Limited June 2006
Task 16 —
Check TP503 Test
Point
If the reference voltage, feedback voltage, and DIG SYN FAST line are all
correct, check the voltage at the
TP503 test point.
1. Measure the voltage at the
TP503 test point (see Figure 9.8).
The oscilloscope should show a DC level less than 3.0V with no sign
of noise or modulation.
2. If the correct result is obtained, go to Step 3. If it is not, go to Step 5.
3. The loop filter is faulty but the above measurements do not provide
more specific information. Check IC504, IC505, Q511
(see Figure 9.4) and associated components.
4. Check the following components (see Figure 9.8), then conclude
with Step 9:
Note On early issue boards, C548 may be fitted instead of these
components.
5. Remove R566 and R570 (see Figure 9.8), which provide a
modulation path to the VCO(s).
6. Repeat the measurement of Step 1.
7. If the correct result is now obtained, go to Step 8. If the correct result
is still not obtained, the CODEC and audio circuitry is suspect;
resolder R566 and R570 in position (see Figure 9.8), and go to
“CODEC and Audio Fault Finding” on page 381.
8. Resolder R566 and R570 in position (see Figure 9.8).
9. Check IC504 (pins 6, 8, 9) (see Figure 9.4) and the associated
components in the loop filter.
10. If a fault is found, repair the circuit, and confirm that the voltages are
now correct. If they are, go to “Final Tasks” on page 157. If they are
not, or if no fault could be found, replace the board and go to “Final
Tasks” on page 157.
TP503 test point: < 3.0 V DC
A4 band: C5085 to C5089
B1 band: C5085 to C5089
C0 band: C5085 to C5088
D1 band: C5085 to C5088
H5 band: C5085 and C5086
H6 band: C5085 and C5086
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